The invention relates to a buttonhole frame arranged to be attached to an adjustment mechanism, in particular an embroidery frame drive mechanism, of a domestic sewing machine, the frame having a frame plate and a clamping element, which is movable in respect to the latter, each of which has passages for the sewing machine needle, wherein the frame plate and the clamping element are arranged to clamp the material to be sewn between them with the aid of a pressing element.
Holders for material to be sewn are known in general from U.S. Pat. No. 3,664,288. There, a clamping element, which is pivotable around a hinge, can be pressed against a frame plate by means of a locking lever. However, the described structure is very bulky and is not easily suitable for domestic sewing machines, for example.
DE 40 08 148 C2 describes a clamping element of a folding device for material to be sewn, which is not intended to be used with a domestic sewing machine. The static arrangement also includes a large-size mechanism for lifting the clamping element.
DE 34 09 942 C1 describes a clamping mechanism for material to be sewn specifically for buttonhole sewing machines, wherein the material to be sewn is pressed against a contact plate with the aid of a clamping mechanism.
DE 41 13 131 C1 and DE 1 990 644 U describe elastic supports in a clamping area.
It is the object of the present invention to create a buttonhole frame suitable for use with domestic sewing machines.
In accordance with the invention, this object is attained by a buttonhole frame of the type mentioned at the outset, which further comprises a wedge surface which, in the course of the movement of the pressing element into the position where it clamps the material to be sewn, builds up a pressing force on the clamping element against the frame plate, and a restoring spring, which lifts the clamping element off the frame plate in the opened position of the pressing element.
Along with a space-saving arrangement of the whole and a short displacement path of the pressing element, the use of the wedge surface permits an assured holding of the material to be sewn in a defined position. The pressing forces built up by the wedge surface in normal direction in respect to the displacement direction assure a particularly good holding of the material to be sewn in the buttonhole frame, so that the latter cannot slide in the course of processing, for example in the course of sewing a buttonhole.
The restoring spring which, in comparison with separate adjustment mechanisms, also is space-saving, makes manipulation easier since, following the pushing, or pivoting the pressing element back, the clamping element is automatically lifted off the holding plate by the action of the restoring spring, so that the material to be sewn can be easily removed.
In a preferred embodiment of the invention it is provided that the wedge surface is embodied as a pressing element on a longitudinally displaceable slide, and works together with a stationary abutment formed on the holder for the material to be sewn.
With an embodiment of this type, where the slide acts indirectly or directly on the clamping element, the material to be sewn is clamped in the buttonhole frame by a simple linear displacement, and the required holding forces are built up by means of the displacement. Alternatively to this structure it is conceivable to embody the wedge surface on a rotatably seated pressing element, for example in the form of a screw thread, wherein the pressing force is built up by a rotating movement of the pressing element into the clamping position.
Independently of the structure of the pressure element, it is particularly preferred to design the inclined surface in such a way that self-locking of the pressing element occurs and/or a flattening in the area of the end position is provided. In this way it is made possible without an increased structural outlay to assure a secure holding of the pressing element in the clamping position, and therefore an operationally reliable clamping of the material to be sewn in the buttonhole frame.
An embodiment is particularly practical, wherein the pressing element acts on the pivotably seated clamping element. Pivotable seating of the clamping element permits the simple working together with the pressing element which, during the displacement, is directly pushed by the wedge surface against the clamping element, or a pivot arm, for example, which is connected with the clamping element.
In order to be able to optimally match the surface of the passages to the locations to be processed of the material to be sewn, it is provided in a preferred further development of the invention that the frame plate and the clamping element are exchangeably arranged in pairs on the holder for the material to be sewn, wherein each pair has congruent passages.
It is possible in this way to select passages by means of a matched pair consisting of a frame plate and a clamping element which are embodied as accurately as possible in such a way that the needle entry into the material to be sewn takes place near the edge of the passages, so that no variations in the dimensions can occur, even with delicate material to be sewn, which tends to warp. For example, the passages can be designed selectively in a vertical or horizontal arrangement corresponding to the shape of a buttonhole to be sewn.
In order to be able to better compensate different thicknesses of the material to be sewn, a layer of a soft, elastic material, for example sponge rubber, is provided between the frame plate and the clamping element of a further preferred embodiment. The sponge rubber layer is here suitably provided on the contact surface of the clamping element in order not to hinder the insertion and removal of the material to be sewn.
With comparatively thick material to be sewn, the sponge rubber layer can be easily compressed without the pressing forces being increased to a considerable amount, which could possibly result in damage to the material to be sewn when using delicate materials. On the other hand, the sponge rubber layer also provides an assured holding of the material to be sewn in the holder, even with thin materials.
To make the alignment of the material to be sewn easier for the user, at least one stop for the material to be sewn is usefully provided, which defines a set distance from the passages, wherein an adjustable distance of the stop from the passages for adaptation to different sizes is particularly preferable. A scale which shows the distance between the center of the passages and the set position of the stop is a further preferred embodiment for making the manipulation of the holder easier for the user.
An exemplary embodiment of the invention is explained in greater detail in what follows, making reference to the attached drawings.